Electrographic recording medium



I Patented J... 9, 1945.

' UNITED.

STATES PATENT OFFICE nmc'mocmrmc RECORDING MEDIUM Robert B. Glbney,Basking- Ridge, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York No Drawing.Application February 6, 1941,

Serial No. 377,760

-- 7 Claiins. (01. 204-2) such media led to the attempt to find onewhich shall be highly sensitive to electric current, pro-' ducing easilyrecognizable marks when activated by a comparatively weak current; whichshall be capable of producing clear marks of high definition with aminimum of fringes; which shall produce marks which are stable andpersist over reasonable times; which shall give marks of a pleasingcolor on a back-ground of equally p as having all of the desirablecharacteristics above.

mentioned. In 'a somewhat narrower aspect media are provided in whichone or more of these characteristics are absent or are present in alesser degree than in the specific preferred form but which arenevertheless improvements over the media heretofore used, as willhereinafter appear.

Other objects and features will be apparent from the followingdescription of the invention.

To overcome the various disadvantages and objections which characterizethe well-known electrographic papers of the inorganic salt type it hasalready been proposed to employ a paper impregnated with leuco bases, e.g., tetramethyl-p-diamino triphenyl methane and to produce adiscoloration reaction therein on the passage of electric current byanodic oxidation to the corresponding dyestuff, e. g. malachite green.So far as is known, and as pointed out in U. S. Patent 1,970,- 539,August 21, 1934, papers so prepared have been unsatisfactory inasmuch asthey either show a tendency to autooxidation or else the coloredreaction products are themselves unstable, causing the electrographicrecord to disappear.

In contrast with the foregoing, however, it has group of dye-formingcompounds known as leuco bases lends itself readily to this use, andalso that many such compounds, and the one preferred in particular, maybe admixed with certain other carefully chosen ingredients which add totheir stability and generally to their reliability in operation and use.

Accordingly, and in pursuance of the objects of the invention in itspreferred form ther is prepared an alcohol solution of an organic leucobase,

preferably a hydrocarbon substituted diaminodiphenylmethane, such astetramethyldiaminodiphenylmethane, a weak, normally solid acid which isslightly soluble in water, such as boric acid, a substance such aspotassium hydroxide, capable of forming a buffer by equilibrium reactionwith the acid, and a hygroscopic agent such as sorbitol. In a preferredsolution the constituents and their proportions are as follows:

. The preferred base is a strong white bond paper which shall not tearwhen wetted to saturation, but this preference is based only on thestrength of the paper, and any paper, or indeed any weblike materialcapable of absorbing or otherwise carrying and holding the novelmixture, may be employed without departing from the spirit of theinvention. The paper, if paper is used, may be impregnated with themixture in any desired manner, for example by passing it in theform ofa. long strip through a vat or tub containing the mixture while pressingand rolling it to remove any air bubbles which may be attached to it andto ensure a thorough soaking. This method has the advantage that themanufacturing process for the paper itself is not modified or altered inany way. However, if desired the impregnation may be carried out as apart of the paper-making process and at any point therein at whichsubsequent operations willnot disturb or be disturbed by the presence ofany of the ingredients of the mixture.

been discovered that a particular member of the' Indeed, if properprecautions be observed, the

novel solution may be mixed with the batch itself prior to the feltingoperation. I

' It is, of course, advisable to see that the novel mixture be athorough and intimate one, else the resulting treated paper might benon-homogeneous in its constituents and therefore non-uniform in it prperties.

After the paper has been thoroughly saturated with the novel solution inorder to secure an intimate and uniform dispersion of all theingredients, the alcohol should be allowed to evaporate or otherwiseremoved, leaving dispersed in the paper anintimate and homogeneousmixture of substances all of which are solids, with the exception of asmall quantity of moisture taken up from the atmosphere by the sorbitoThe paper is thus dry enough to be handled with safety without having toresort to especial precautions to avoid injury to it as might be thecase were it soaking wet.

The removal of the alcohol should be complete, else the colored depositformed by the passage of the current might spread and so degrade thedefinition of the marks, since the dye, as

well as the leuco base, is soluble in alcohol. However, the presence ofthe moisture absorbed by the sorbitol in the proportions given abovedoes not produce this effect since the organic compound is notappreciably soluble in water.

Since the novel mixture is practically color-' less the natural color ofthe paper is preserved, and this provides the background for themarkings. For most purposes plain white paper is to be preferred, but ifdesired a colored paper may be employed provided the color is oneagainst 'which the color of the dye formed in use will provide goodvisual contrast. For example, the blue dye oftetramethyldiaminodiphenylmethane is clearly visible against thebackground of a scarlet paper. 1

The paper as thus prepared is slightly sensitive to bright light,- butnot so much so as to necessitate especial precautions in its handling.For example, it may safely be kept in a roll or in the drawer of acabinet or desk, and may be unrolled I alcohol, are all non-volatile,the composition of the impregnating mixture in the paper remainsunchanged and therefore the properties of the impregnated paper remainunchanged also.

When the paper as thus prepared is to be put to its intended use it maybe placed between the electrodes which are to carry the activatingcurrent. For example, it may be passed between the exposed tips of aplurality of conductors which extend through the end wall of a cathoderay tube V and a knife edge anode in registry therewith, as

' disclosed in a copending application of E. Bruce,

Serial No. 328,816, filed April 10, 1940 (Bruce 28) now Patent No.2,278,433, Feb. 17, 1942, and advanced by rollers in a directionperpendicular to the row of conductors while the cathode beam plays fromend to end of the row. As the beam strikes a particular conductor acurrent flows from it through the paper, which is/rendered conductive bythe presence in it of the electrolyte, to the external knife edge anode.In the presence ofthe weak acid oxidation of the leuco base takes placeat the anode resulting in'the iormation of ter. In an actual test witha. cathode ray tube whose conductor tips were about 1.5 mils square,spaced about 10 mils apart, and an anode whose knife edge was about 10mils wide, and through which a current of 500 microamperes was passedfor 1/2000 second, the novel paper, prepared withtetramethyldiaminodiphenylmethane, gave good, strong, well defined marksof a deep blue color. In this case the discharge of electricity was ofthe order of A microcoulomb' and the momentary current density at theanode during discharge was of the order of 5 amperes per square inch. Toavoid electrolytic reaction with the materials of the comb conductors onthe knife edge anodes, the latter are preferably of non-corrosive metal.

After the passage of the current, the process is complete, the pap rrequiring no finishing process in the nature of development or fixation.Furthermore, the dark marks are sumciently durable for all practicalpurposes. It is only necessary to preserve the marked paper in such amanner.that it will not be contacted by a dye solvent, such as alcohol,which might cause the marks to run, and to preserve it from excessiveexposure to light which might darken the ight background.

Although a paper bearing the organic substance alone will, whensufliciently wetted to ren der it conductive, become discolored on thepassage of current, each of the auxiliary ingredients of the novelmixture offers specific advantages. 'I'he oxidation reaction takes placebest in the presence of a Weak acid, especially when stabilized by abuffer. In the presence of a strong acid the paper is unduly lightsensitive and the strong acids cannot readily be stabilized. Boric acidmeets these requirements, being a weak, readily stabilized acid. Itoffers the further advantage that it is normally solid and notdeliquescent so that the paper containing it is not rendered wet by theacid or by atmospheric moisture absorbed by the acid. The only moisturein the paper is that absorbed by the sorbitol, and its quantity, whichneed only be small, is therefore controllable and independent of theamount of acid present. v

The potassiumhydroxide reacts with the boric acid to produce potassiumborate which exists in equilibrium with the boric acid and the potassium hydroxide. The borate acts, along with the acid, as an electrolyteto conduct the current through the paper, and it also serves as a.bufier to maintain the proper ion concentration to allow the oxidationreaction to take place.

' A further advantage offered by the potassium borate and therefore bythe potassium hydroxide from which it is formed by reaction with theboric acid, is that its anions in electrolysis are not discharged at ananode in preference to oxyan organic dye of deep color and lastingcharac- 76 gen. Therefore the .concentrations of the ingradients are notsubstantially altered in the course of the reaction, nor is the anodecorroded or the paper discolored by the presence of secondary reactionproducts.

It is also true of the novel mixture as a whole thatthe' oxidationreaction takes place without either cathode or anode taking partchemically therein. 'As a result both electrodes may be incorporated aspermanent elements of the recording apparatus.

The sorbitol acts to absorb and hold suflicient moisture to dissolve asmall quantity of the boric acid and the potassium hydroxide and sopermit the electrolytic action to take place. The alcohol is employedbecause it .is believed to be the best aso'aus known solvent for boththe organic leuco base and the boric 'acid. The choices of potassiumborate for the electrolytic salt and sorbitol for v the hydroscopicagent are further dictated by the facts that these substances too aresoluble in alcohol.

The considerations underlying the choice of the preferred proportions ofthe novel solution are as follows: About grams of the ,orgaiiicdyeforming substance per liter of alcohol will impregnate paper ofordinary thickness suillciently to give strong, clear marks when fullyoxidized. Much less than 5 grams per liter would create a danger that,if the distribution of the organic substance in the paper were notcompletely uniform, certain regions of the. paper might not be fullysupplied. To ensure against this, it is preferredto provide somewhatmore, so that the most poorly impregnated regions have. enough while themost fully impregnated regions have a harmless excess. At the same timeit is preferred not'to attempt to use much more than grams per liter ofalcohol on account of the difficulty of dissolving a greater quantity.

The om'dation reaction takes place best in a weak acid medium, and it isdesirable that the boric acid be substantially in excess of thepotassium hydroxide so that the latter may be wholly neutralized withthe formation of potassium borate in sufficient quantities to providethe buffer action. For this purpose 5 to 10 grams of potassium hydroxidehave been found sui'hcient, and the use of a quantity of acid ten timesas great assures that this reaction shall be substantially complete.Such a quantity, namely, 50 to 100 grams of the acid can be readilydissolved in a liter of the alcohol, whereas much more could not be.

50 to 100 grams of sorbitol per liter of alcohol provides a sorbitolconcentration which .is adequate to absorb from the atmosphere an amountof molsturewhich dissolves enough of the acid to render'the paperconductive and allow the electrolytic' action to take place, but not somuch as to wet it enough to render handling difllcult.

Diphenylmethane compounds are preferred to mono-, triortetraphenylmethane compounds for the following reasons. The last namedare so stable that they do not give an appreciable discolorationreaction on the passage of current. The triphenyl compounds areextremely sensitive to light and precautions must be taken to protectthem from exposure, both before the record ing operation and afterward.Therefore, they are believed to be less suitable than theprefen'edcompound. However, they may be found useful in special cases, forexample when only a small current is available and precautions againstexposure to light are not objectionable.

The amino derivative of the monophenyl compound, otherwise known astoluidine, besides being comparatively insensitive to the electrolyticreaction, is self-oxidizing and becomes discolored in time. Furthermore,this substance is corrosive to the hands.

.Each phenyl group may have attached to itone amino group, more thanone, or none at all. Of these combinations it is found that all thecompounds having more than one amino group per phenyl group requireadditional protection in order that the oxidation reaction may progressin a satisfactory manner, and consequently they are not recommended.Likewise, the presence of at least one amino group in the compoundaphygroscopic agent.

group for monophenyl (toluidine), one or two amino groups for diphenyl,and one, two or three amino groups for triphenyl.

Toluidine' and monoaminodiphenylmethane give faint colors of inferiorcharacter and, though their use is possible when sufllciently strong andprolonged currents are available, they are less sensitive than thepreferred compounds.

Each amino group has connected to it two groups, one or both of whichmay be either hydrogen or a hydrocarbon radical. Of the compounds soconstituted all are believed to be operative within the limits discussedabove. In general, a stronger, deeper color is obtainable with acompound in which the replaceable hydrogen of the amino groups are allsubstituted with hydrocarbon radicals than with compounds not sosubstituted. Thus, for example, the tetramethyl compound is preferred tothe tri-, di-, and monomethyl compounds and also to the unsubstitutedcompound, diaminodiphenylmethane. This particular compound offers thefurther advantage that it is readily obtainable in commerce.

Whatever the leucobase employed it is of advantage to employ with it aweak solid acid and a Further, to insure that the light sensitivity.shall not, be excessive, it is desirable to employ in addition either abuffer or.

some compound which will form a bufier by reaction with the acid. Theexact identity and proportions of the ingredients named above serve,

these purposes well, and are therefore recommended, though substanceshaving like or equivalent properties may be employed without departingfromthe spirit of the invention.

While in the foregoing the invention has been described, for the sake ofparticularity, with reference to a sheet support capable of beingimpregnated, it is to be understood that it is equally applicable to aweb-like or film-like material bearing the novel mixture on its surfacebut not impregnated therewith.

What is claimed is:

1. An electrographic porous recording medium in sheet form carrying inits pores a mixture of a compoundselected from the group consisting ofdiaminodiphenylmethane, monomethyldiaminodiphenylmethane,dimethyldiaminodiphenylmethane. trimethyldiaminodiphenylmethane, andtetramethyldiaminodiphenylmethane, an excess of a weak, solid, slightlywater-soluble acid which is not changed in electrolysis, and anelectrolytically inactive hygroscopic agent, said acid and hygroscopicagent being present in such amounts that if humid air is ermitted topenetrate the mixture in said pores a small amount of moisture isabsorbed by the hygroscopic agent and a correspondingly small amount ofthe acid is dissolved in said absorbed moisture, said solution existingin equilibrium with the undissolved excess of acid.

2. The combination with a porous supporting medium, adapted to holdsolid material within its pores when deposited there from solution whichmaterial is suitable for electrographic recording by reason of its beingresponsive to oxygen releasedfrom the anode when a voltage is applied toelectrodes in contact with the medium, of a.

readily volatizable liquid within the pores of said 7 medium in whichhas been dissolved boric acid,

pears to be essential to the formation of the,"

potassium hydroxide and tetramethyldiaminodiphenylmethane, the ratios ofthe weight of the boric acid to that of thetetramethyldiaminohygroscopic substance is sorbitol.

4. The combination with a porous supporting medium adapted to hold solidmaterial within its pores when deposited there from solution whichmaterial is suitable for electrographic recording by reason of its beingresponsive to oxygen liberated at the anode when a voltage isapplied toelectrodes in contact with said medium, of material within the pores ofsaid medium comprising alcohol in which has been dissolved the followingsubstances in the amounts specified -per liter of alcohol; .5 to 10grams of tetramethyldiaminodiphenylmethane, 50 to 100 grams of boricacid, 5 to grams of potassium hydroxide and 50 to 100 grams of sorbitol.

5. The combination with a porous supporting medium of solid materialwithin the pores of said medium adapted when in a substantially drystate for electrographic recording by reason of its being responsive tooxygen liberated at the anode when a voltage is applied to electrodes incontact with said medium which material comprises essentiallytetramethyldiaminodiphenylmethane, ,boric acid, potassium hydroxide,potassium borate and a quantity of hygroscopic substance sufficient tomake the material in its pores a good conductor of electricity when themedium is subsequently exposed to humid air.

6. The combination with a porous supporting medium adapted to hold solidmaterial within its pores when deposited there from solution whichmaterial is suitable for electrographic recording by reason of its beingresponsive to oxygen released from the anode when a voltage is appliedto electrodes in contact with the medium, of a readily volatizableliquid within the pores of said medium in which has been dissolved boricacid, potassium hydroxide and an organic dye-forming substance selectedfrom the group consisting of diaminodiphenylmethane,monomethyldiaminodiphenylmethane, dimethyldiaminodiphenylmethane,trimethyldiaminodiphenylmethane, and tetramethyldiaminodiphenylmethane,the ratio of the weight of the boric acid to that of the dyeformingsubstance and to that of the potassium hydroxide being within the rangefrom five to twenty and the ratio of the weight of the dyeformingsubstance to that of the potassium hydroxide being in the range fromone-half to two, and a quantity of hygroscopic substance sufiicient onlyto make the material in said pores a good conductor of electricity whensaid volatizabie liquid has been evaporated and the medium issubsequently exposed to humid air.

7. The combination with a porous supporting medium adapted to hold solidmaterial within its pores when deposited there from solution whichmaterial is suitable for electrographic recording by reason of its beingresponsive to oxygen released from the anode when a voltage is appliedto electrodes in contact with the medium, of a readily volatizableliquid within the pores of said medium in which has been dissolved anorganic dye-forming substance selected from a group consisting ofdiaminodiphenylmethane, monomethyldiaminodiphenylmethane,dimethyldiaminodiphenylmethane, trimethyldiaminodiphenylmethane, andtetramethyldiaminodiphenylmethane together with boric acid, potassiumhydroxide, potassium borate, and a quantity of a hygroscopic substancesuflicient only to make the material a good conductor of electricitywhen said volatizable liquid has been evaporated and the medium issubsequently exposed to humid air.

ROBERT E. GIBNEY.

